1. Field of the Invention
The present invention generally relates to position and velocity sensors and, more particularly, to a position sensor which provides a selected output when the sensor is at any one of a plurality of selected positions, and to a sensor which provides both position and velocity indicating signals.
2. Description of the Prior Art
In practically all position servo systems which are employed to position an element at any one of discrete positions, a position sensor and a velocity sensor are used. Typically, when the element is moved from one position to another, a position error signal is generated by the position sensor, which is used in driving the element to the desired position. As the element moves to the desired position, a velocity signal, indicative of the element's rate of movement or velocity is generated. It is used as a damping signal to control the element to reach the desired position and be maintained thereat without oscillation until a command is provided to move the element to a different position.
One type of a position sensor which has been used in the prior art includes a linear voltage differential transformer (LVDT). Typically, such a sensor generates an electrical signal, proportional to the displacement of the transformer core with respect to a center position, in response to an external physical motion. The transformer core is generally of high permeability material, e.g., bonded iron powder, which couples AC signals induced in the transformer primary into secondary windings of the transformer. The secondary windings are interconnected so that the AC signals induced in them are 180.degree. out of phase. One example of a prior art LVDT is shown in U.S. Pat. No. 3,600,669.
As long as the transformer core is in its center position, so that the two AC voltages, induced in the two secondary windings, are of equal amplitude but 180.degree. out of phase, the LVDT AC output is of zero amplitude. As the transformer core moves away from the center position the LVDT output amplitude increases from zero. It is related to the core position with respect to its center position. The phase of the LVDT output with respect to the phase of the AC signals in the primary winding is related to the displacement direction. Generally, the LVDT AC output is converted to a DC output signal whose amplitude and polarity are related to the LVDT AC output amplitude and phase, respectively. This DC output signal is the one generally referred to as the position signal. In a typical LVDT the output amplitude is only linearly related to the displacement to within several percents. In the above-referred-to patent, means are described to reduce the electrical output error over a displacement range from the null position to a maximum displacement position. The described LVDT provides only a position signal. However, it does not provide a signal indicative of the rate of movement or velocity of the transformer core.
In the prior art various velocity sensors or transducers, generally referred to as tachometers, are known. One example of a sensor designed to measure linear velocity of a moving element is described in U.S. Pat. No. 2,764,702. The described transducer is one in which a voltage is induced in a coil as a function of the rate of movement or velocity of magnetic-flux-providing means with respect to the stationary coil. Typically, a velocity transducer provides only an output signal indicative of velocity and does not provide any indication of the position of the magnetic-flux-providing means with respect to the coil.
Generally, prior art velocity sensors provide only a velocity signal, and position sensors provide only a position signal, in any application in which both velocity and position signals are required, e.g., a position servo system, two separate sensors must be provided. This is quite undesirable since the need for two separate sensors increases the overall cost of the servo system. Also, the two separate sensors increase the system size which in some cases may be most undesirable particularly where the space where the sensors are to be located is very limited. A need therefore exists for a small sensor capable of providing both position and velocity signals. Also, in applications in which an element is to be positioned at any one of discrete positions, a need exists for a relatively simple and reliable system for accurately positioning the element at any one of the desired positions.